Lockin thermography (or LT) is based on thermal waves generated inside the specimen under study by submitting it to a periodic (sinusoidal) thermal stimulation. In the case of a sinusoidal temperature stimulation of a specimen, highly attenuated and dispersive waves are found inside the material (in a near surface region). These waves are known as “thermal waves.” This is not a new concept since these thermal waves were first investigated by Fourier and Angström back in the XIX century. Of interest, is that these waves can be generated and detected remotely, for instance by periodically depositing heat on the specimen surface with a lamp. This is called photo-thermal lockin thermography.

The lockin terminology refer to the necessity to monitor the exact time dependence between the output signal and the reference input signal (i.e. the oscillating - also called modulated - heating). The resulting oscillating temperature field (following the oscillating thermal stimulation) in the stationary regime (that is after the transient regime) is remotely recorded through its thermal IR emission. A proper experimental apparatus allows to observe the amplitude and phase of the resulting thermal wave on the specimen.

The thermal images obtained from such apparatus are different than other thermographic images by many aspects since both phase and amplitude images are available. In basic PT, thermographic images are obtained. These images correspond to a mapping of the emitted thermal IR power while phase images are related to the propagation time and the amplitude images are related to the thermal diffusivity. For many NDT applications, a strong point of LT is the phase image which is relatively independent of local optical and thermal surface features.

The depth range of amplitude image is roughly given by thermal diffusion length µ expressed by:

µ=sqrt(2k/ωρc)

with thermal conductivity k, mass density ρ, specific heat c and modulation frequency ω.

In the case of phase images, it has been reported that depth range is about twice as large. In the equation, it is seen µ is related to the inverse of ω, this means a low modulation frequency will probe deeper (as in ultrasonics).

LT concepts can be deployed in a point-scanned laser fashion but also on a full-field basis by illuminating the whole sample periodically with a lamp (to generate the thermal wave) while the signal pickup is performed by an IR camera and associated equipment. In fact, the relaxed heating constraints associated with the phase image enables the inspection of large surfaces, up to several square meters, provided the spatial resolution of the IR camera is high enough. Interestingly, it is relatively easy to deposit the modulated heating over a surface using lamps.

More recently, it was shown that a suitable thermal stimulation can also be obtained using an ultrasonic transducer (shaker) attached to the specimen (conversely, the specimen can be partly immerged into an ultrasonic bath). In this case, the high frequency ultrasonic signal (typ. 40 kHz) is modulated with a low frequency signal. This low frequency modulation creates a thermal wave of desired wavelength as in conventional LT while the high frequency acts as a carrier delivering heating energy right inside the specimen. This technique is also known to as the ‘loss angle lockin thermography’ and it is reported to detect deeper and smaller defects while the selective heating allows a better discrimination among detected defects. Typical applications are for detection of corrosion, vertical cracks and delaminations.